In recent years, plant growth using artificial light sources has been the subject of much research. In particular, cultivation methods using illumination from light-emitting diodes (LED), which exhibit excellent monochromaticity, provide favorable energy conservation and long life, and can be easily miniaturized, are garnering much attention.
Based on the results of research to date, one emission wavelength band that has been confirmed as a suitable light source for plant growth (photosynthesis) is red light having a wavelength within the region from 600 to 700 nm.
Examples of conventional red light-emitting diodes of this wavelength range that have been investigated include those having light-emitting layers composed of AlGaAs and InGaNP and the like (for example, see Patent Documents 1 to 4).
On the other hand, compound semiconductor LEDs having a light-emitting layer composed of an aluminum-gallium-indium phosphide (composition formula: (AlXGa1-X)YIn1-YP, wherein 0≦X≦1 and 0<Y≦1) are also known.
Among these LEDs, a light-emitting layer having the composition Ga0.5In0.5P exhibits the longest wavelength, and the peak wavelength obtained from this light-emitting layer is in the near 650 nm. Accordingly, achieving practical application of, and a high level of brightness for, compound semiconductor LEDs in the region to the long wavelength side of 655 nm has proven problematic.
A light-emitting unit having a light-emitting layer composed of (AlXGa1-X)YIn1-YP (wherein 0≦X≦1 and 0<Y≦1) is formed on a monocrystalline substrate of gallium arsenide (GaAs).
At this time, the composition of the above-mentioned light-emitting unit is selected so as to match the lattice constant of the GaAs monocrystalline substrate.
On the other hand, although investigations have been conducted into light-emitting layers having strain for laser devices having a different light emission mechanism, there are currently no practical applications of strained light-emitting layers in the field of light-emitting diodes (for example, see Patent Document 5).
Furthermore, investigations are also being pursued into light-emitting diode light-emitting units that utilize a quantum well structure (for example, see Patent Document 6). However, because the quantum effect obtained by utilizing a quantum well structure shortens the emission wavelength, this effect has been unable to be applied to techniques requiring wavelength lengthening.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 9-37648
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-27831
Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2004-221042
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2001-274454
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2000-151024
Patent Document 6: Japanese Patent (Granted) Publication No. 3,373,561